The assembly of the wild type P22 procapsid requires the participation of approximately 300 molecules of scaffolding protein in addition to the 420 coat protein subunits. In a mutant P22 in which the scaffolding protein is absent, the P22 coat protein can assemble both wild-type and smaller sized closed capsids. Both sizes of procapsid assembled in the absence of the scaffolding protein have been studied using electron cryo-microscopy. The structural studies show that the larger particles have T=7 icosahedral lattices and appear the same as wild-type procapsids. The smaller particles possess T=4 icosahedral symmetry. Both procapsids consist of very similar penton and hexon clusters except for an increased curvature present in the T=4 hexon. In particular, the pronounced skewing of the hexons is conserved in both sizes of procapsid. The T=7 procapsid has a local non-icosahedral two-fold axis in the center of the hexon and thus contains four unique quasi-equivalent coat protein conformations which are the same as those in the T=4 procapsid. These structural results have led us to propose that the scaffolding proteins are used to regulate the size of the procapsid rather than to provide a template for its assembly.